1. Field of the Invention
The present invention relates to a development apparatus suitable for an electrophotographic copy machine, a laser beam printer or the like and, more particularly, to a development apparatus which develops a dot latent image formed on an image carrier by magnetic brush of a two-component developer that consists of a magnetic carrier and a toner, and an image forming apparatus provided with such a development apparatus.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus or an electrostatic recording image forming apparatus is widely used as an image forming apparatus such as a copy machine, a printer or a facsimile machine. Such an image forming apparatus forms an electrostatic latent image on an image carrier that consists of a belt-like photo-conductor or the like.
The electrostatic latent image is transformed into a visible image by a development apparatus, and the visible image (for example, a toner image) is transferred onto a record paper. The development apparatus, which visualizes the electrostatic latent image on the image carrier, requires an easy transfer capability, a half-tone reproducibility, a development property stability with respect to temperature and humidity, etc.
There is suggested a so-called contact-type two-component development system, which satisfies such a demand. The contact-type two-component development system performs a development by using a two-component developer by causing magnetic brush to slide on an image carrier surface, the magnetic brush consisting of a toner and a magnetic carrier. That is, in the contact-type two-component development system, the two-component developer is conveyed to a development area facing the latent image carrier in a state where the developer is in a spicate form like a brush chain standing on the latent image carrier so that the toner in the developer is supplied to an electrostatic latent-image part on the latent image carrier.
The development apparatus using the contact-type two-component development system has a problem in that the toner concentration in the developer must be controlled and an agitation mechanism for the developer is needed, which increases the size of the apparatus. However, the development apparatus using the contact-type two-component development system is superior to other apparatuses in obtaining a high image quality, easy conveyance of developer, etc, and, thereby, many development apparatuses use the contact-type two-component development system.
FIG. 1 is an illustrative cross-sectional view of the above-mentioned contact-type two-component development apparatus.
The development apparatus shown in FIG. 1 comprises a development roller 41 having a magnetic roller 44 and a development sleeve 43 rotatable around the magnet roller 44. The magnet roller 44 is provided with a plurality of magnetic poles, and is formed in a cylindrical shape. A development area of the developer carrier surface of the magnet roller 44 is provided with development main magnetic pole which causes the developer to stand in a spicate or ear-like form. A doctor blade 45 and a screw 47 are provided around the sleeve 43. The screw 47 supplies the two-component developer to an outer surface of the sleeve 43. The doctor blade 45 controls the thickness of the developer layer of the two-component developer adhering on the outer surface of the sleeve 43. Thereby, the developer layer having an appropriate thickness is formed on the sleeve 43.
The developer layer forms so-called magnetic brush, which is formed by the magnetic carrier extending in a spicate or ear-like form along magnetic field lines formed by the magnet roller 44. The developer extending from the outer surface of the sleeve 43 moves when at least one of the sleeve 43 and the magnet roller 44 rotates. The developer conveyed to the development area extends along magnetic field lines generated by the development main magnetic pole, and the magnetic brush of the developer contacts a surface of a latent image carrier 1 in a position facing the latent image carrier 1 so that the magnetic brush of the developer wipes the surface of the latent image carrier 1. The magnetic brush in contact with the surface of the latent image carrier 1 supplies a toner to the latent image carrier 1, while rubbing a portion where an electrostatic latent image is formed, based on a difference in the relative linear velocity between the sleeve 43 and the latent image carrier 1.
However, in the above-mentioned development apparatus, it is difficult to simultaneously establish a development condition for increasing the image concentration and a development condition for achieving a low-contrast image. That is, it is difficult to improve both a high-concentration section and a low-concentration section simultaneously. As for the development condition to increase an image concentration, it is considered to 1) reduce a development gap, which is a distance between the latent image carrier and the developer carrier or 2) increase a width of the development area. On the other hand, as for the development condition to achieve a low-contrast image, it is considered to 3) increase the development gap or 4) decrease the width of the development area. That is, the development conditions are opposite to each other and are incompatible with each other, and it is generally difficult to achieve a good image by satisfying both conditions over a whole concentration range. For example, if an importance is given to a low-contrast image, the image concentration cannot be made high. Moreover, a so-called “rear end missing” tends to occur, which is a white part formed at the end of a cross part of solid lines, a black solid part or a halftone solid image. Furthermore, there may occur a phenomenon in that a horizontal line of a grid image formed by the same width is thinner than a vertical line, or a small point image such as single dot image is not developed.
In order to solve the above-mentioned problems, there is suggested a method of setting a main pole angle of the magnet roller to an upstream side. Additionally, Japanese Laid-Open Patent Application No. 07-140730 suggested a method of giving a fixed relation between a distance between a regulation member (a member which regulates an ear height of the magnetic brush) and the development sleeve and a distance between the belt-like photo-conductor and the development sleeve.
Moreover, an improvement in the development capability of the two-component development apparatus is also a subject to be achieved. Various methods have been suggested to achieve the improvement in the development capability.
Japanese Patent Publication No. 02-59995 discloses a method of improving the development capability by bringing a magnetic pole adjacent to the development main pole closer to the development main pole. According to this method, although the concentration of a horizontal line falls (the same phenomenon as the above-mentioned thinning of the horizontal line), the decrease in the concentration is prevented by weakening the magnetic brush by lowering the saturation magnetization of the carrier. In addition, Japanese Laid-Open Patent Application No. 06-149063 suggests a non-contact two-component development apparatus using a pole arrangement in which the magnetic brush does not contact a photo-conductor. In order to prevent the “rear end missing” caused by a counter charger or a thinning phenomenon of a line image or a point image, it is suggested to reduce a velocity ratio relative to a photo-conductor. However, if the velocity ratio is reduced, an amount of developer supplied to a development nip portion per unit time decreases. Thereby, development capability declines and the fault arises that a sufficient image concentration cannot be obtained.
In a development apparatus disclosed in Japanese Laid-Open Patent Application No. 07-140730, in order to eliminate the thinning of a horizontal line (thin line omission), a ratio of Hcut to Dsd is set within a range of 1.2<Dsd/Hcut<1.6, where Hcut is a distance between a restriction member and a development sleeve and Dsd is a distance between the development sleeve and the belt-like photo-conductor. However, as a value of Dsd/Hcut goes away from 1 (Hcut becomes smaller than Dsd), the density of the magnetic brush is decreased in the maximum proximity part between the development sleeve and the photo-conductor. For this reason, contact of the magnetic brush to the belt-like photo-conductor becomes uneven, and a part, which is not wiped by the magnetic brush, is generated on the photo-conductor. This may cause a phenomenon in which a part of dots becomes small in its size or totally eliminated especially in an isolated dot image (for example, an image in which a dot of 600 dpi is written at intervals of 5-10 pixels). If the isolated dot is not uniformly reproduced, reproducibility of a so-called high-contrast part deteriorates, which results in formation of an image having a high-contrast part with a poor gradation. In addition, in a half-tone image having a concentration about 0.3-0.8 (ID), since the contact of the magnetic brush is uneven, a feeling of roughness is enhanced, which causes deterioration of image.
Moreover, there is a problem shown in FIG. 1. FIG. 1 is a cross-sectional view of a development area of the conventional image forming apparatus. FIG. 1 illustrates a state where the magnetic brush B contacts with a latent image carrier 1. In FIG. 1, since the height of spikes or ears, the configuration or the density of the magnetic brush B is uneven, there may occur unevenness of concentration or roughness in the developed image. By adjusting an arrangement or a configuration of the doctor blade (developer regulation member), variation in the distance between the spikes or ears of the magnetic brush B can be improved. However, since a magnetic pole exists after passing through the doctor blade and before reaching the development area, it is difficult to align the extreme ends of the spikes or ears of the magnetic brush as shown in FIG. 1. It is considered that the cause of the above-mentioned problem is variation in the grain size of the carrier, variation in permeability, or a number of toners or a state of adhesion differing between carriers.
Japanese Patent Publication No. 02-59995 suggests a method of preventing decease in the concentration of a horizontal line by lowering the saturation magnetization of a carrier. However, when the saturation magnetization of a carrier is lowered, a so-called carrier adhesion may tend to be generated. Moreover, when an amount of electric charge of the toner is decreased so as to prevent the carrier adhesion, an amount of non-charged toner is increased, and so-called background roughness may be generated. Since the technique disclosed in Japanese Laid-Open Patent Application No. 06-149063 relates to a non-contact two-component development, an intensity of a development electric field is low, and it is difficult to raise development capability.
Moreover, in order to acquire an image without unevenness of concentration and roughness of the image, various measures are taken so as to uniformize an amount of developer existing in a development area. For example, in order to densify the ear of the magnetic brush, Japanese Laid-Open Patent Application No. 5-289522 suggests an arrangement of a uniformizing member in the development area for the purpose of increasing the density of the developer in the development area. Japanese Laid-Open Patent Application No. 11-143236 suggests a technique which improves nonuniformity of the thickness of the developer layer after being passed through the developer regulation member due to a decrease in the magnitude of magnetization and an increase in the density of the developer when a small size carrier is used, and which extracts a relational expression between a carrier size and a magnetic pole facing a magnetic field. Furthermore, Japanese Laid-Open Patent Applications No. 8-146757, No. 5-11616, No. 5-158352, and No. 10-10871 suggest a technique using a magnetic material or a magnet as the restriction member so as to control fluctuation in an amount of developer after being passed through the restriction member. Moreover, Japanese Patent publications No. 7-92626 and No. 7-107618 and Japanese Laid-Open Patent Applications No. 5-323792 and No. 10-133481 disclose a technique which prevents fluctuation in an amount of developer after being passed through the restriction member due to fluctuation in a restriction gap between the restriction member and the development sleeve by arranging the restriction member at a position considering a magnetic distribution.
However, there is a problem in that the above-mentioned conventional techniques cannot provide a sufficiently high image quality since they merely control fluctuation in an amount of developer with respect to passage of time or merely increase the density of developer in a development area.
Furthermore, in the above-mentioned development apparatus using the magnetic brush development method, in order to perform uniform development without unevenness in concentration, it is required to maintain a uniform thickness of the developer layer formed on the development sleeve so as to be conveyed to the development area. This is particularly important in the non-contacting development method. With respect to one-component magnetic toner, Japanese Laid-Open Utility Model Application No. 57-79863 and Japanese Laid-Open Patent Application No. 58-21772 suggest a technique to form a magnetic field having a plurality of peaks and provide a developer layer restriction member between the adjacent peaks. However, the above-mentioned technique alone provides little effect with respect to the two-component magnetic developer.